Introduction
The present invention relates to the drying of microspheres, and to the dry expansion of microspheres.
Microspheres are heat expandable thermoplastic polymeric hollow spheres containing a thermally activatable expanding agent. Such materials, the method of their manufacture, and considerable information concerning the properties and uses of microspheres, are all set forth in U.S. Pat. No. 3,615,972, issued to Donald S. Morehouse. Other teachings concerning such materials are found in, for example, U.S. Pat. Nos. 3,864,181; 4,006,273; and 4,044,176.
In following the teachings of the Morehouse patent, the microspheres are made in an aqueous system by the limited coalescence process under pressure, and the resulting product is a "wet cake" of the unexpanded microsphere beads wet with water. The wet cake is typically about 40 to 75 weight percent solids, and because of the wetting agents employed in the formation of the beads, the surface will be wet. Separation of water and beads has not been a simple process.
Many important uses of the microspheres require the removal of the water, to produce dry, free flowing microsphere beads. In many uses, it is also appropriate to pre-expand the beads before use. In unexpanded form, the dry microsphere beads typically have a displacement density of about 1.1. Dry, expanded microspheres typically have a density by displacement of less than 0.06 grams/cc, and are highly useful in the production of syntactic foams in a wide variety of polymer matrices. Dry, free-flowing microspheres, and dry, free-flowing, pre-expanded microspheres have now achieved a commercial recognition and market demand for such uses and others.